The present invention relates generally to circuit protection in motor vehicles, and more particularly to an inexpensive means for eliminating fuse boxes in motor vehicles and the need to funnel circuits needing overload protection to such fuse boxes.
Vehicular wiring must be protected against many potential failure modes. (The terms "vehicular" and "vehicle" are intended to include automobiles, trucks, motorcycles and other apparatus in which electrical splices are used.) One failure mode in vehicles is the possibility that the insulation on any of the many wires in the vehicle will be broken such that the affected wire or wires will short to ground or to other wires or components that may be bare and above ground. The magnitude of the problem is seen in the fact that a modern motor vehicle contains hundreds of circuits using hundreds of feet of insulated wire. Each wire must be protected against faults by using fuses or other circuit protection devices. Such protection devices are sized to handle the sum of the currents through all wires connected to the devices. Further, each wire that carries current provided through its fuse is sized such that the fuse will melt and interrupt before the wire is destroyed. This requires that each wire have a current carrying capability larger than the load connected to the wire requires. This adds to the cost and weight of automotive wiring systems.
There exists in the art self-resettable materials and devices that possess positive temperature coefficient (PTC) characteristics. Such materials and devices are internally structured in a manner that will cause a rise in temperature of the material when excessive current flows through the material. This, in turn, causes the electrical resistance of the material to rise. The rise in resistance reduces the flow of current to a safe condition. Thus, when a fault occurs that causes an increased flow of current sufficient to heat the device, the device increases its temperature and resistance to reduce current flow.
Present technology places PTC devices within switches or loads. These are not optimum schemes because many switches control several different loads. In addition, circuit protection devices require that switches and loads be redesigned to include such devices which involves costly retooling.